Over the last decade, nanoparticles, i.e. particles having sizes below 1 micrometer, have attracted a great deal of interest in research and industry due to their unique properties. Research and development in the optoelectronic area have focused on luminescent particles in view of their possible application in light emitting diodes (LED), displays, optoelectronic devices in nanometer dimensions or as a light source in low threshold lasers.
Among luminescent materials, a distinction is often made between semiconductor and non-semiconductor materials.
Semiconductor nanoparticles (often referred to as “quantum dots”, such as II-VI or III-V semiconductors which may be doped or not, are characterized by a quantum confinement of both the electron and hole in all three dimensions which leads to an increase in the effective band gap of the material with decreasing crystalline size. Consequently, it is possible to shift both the optical absorption and emission of semiconductor nanoparticles to the blue (higher energies) as the size of the nanoparticles gets smaller.
Water-soluble core/shell semiconductor nanocrystals are, for instance, described in WO 00/17655.
If being compared with quantum dots, it constitutes the particular attractivity of nanocrystalline non-semiconductor-based luminescent materials, in particular, lanthanide-doped metal oxides or salts, that their fluorescent emission is relatively narrow and does not depend to a greater extent on the host material and the size of the nanoparticles. It is rather only the type of lanthanide metal which determines the emission color. PCT/DE 01/03433 assigned to the same applicants discloses a generally applicable synthesis method for lanthanide-doped nanoparticles of this type. These nanoparticles can be produced in sizes (below 30 nm) no longer interacting with the wavelength of visible light, thereby leading to transparent dispersions, e.g., in organic or aqueous solvents.
Other publications relating to lanthanide-doped non-semiconductor based luminescent nanoparticles are, for instance:    K. Riwotzki et al.: Angewandte Chemie, Int. Ed. 40, 2001, pages 573-576 with respect to LaPO4:Ce,Tb;    K. Riwotzki, M. Haase, J. Phys. Chem. B; Vol. 102, 1998, pages 10129-10135 with respect to YVO4:Eu, YVO4:Sm and YVO4:Dy;    H. Meyssamy, et al., Advanced Materials, Vol. 11, Issue 10, 1999, pages 840-844 with respect to LaPO4:Eu, LaPO4:Ce and LaPO4:Ce,Tb;    K. Riwotzki et al., J. Phys. Chem. B 2000, Vol. 104, pages 2824-2828, <<(Liquid phase synthesis doped nanoparticles: colloids of luminescent LaPO4:Eu and CePO4:Tb particles with a narrow particle size distribution>>;    M. Haase et al., Journal of Alloys and Compounds, 303-304 (2000) 191-197, “Synthesis and properties of colloidal lanthanide-doped nanocrystals”;    Jan W. Stouwdam and Frank C. J. M. van Veggel, Nano Letters, ASAP article, web release May 15, 2002, “Near-infrared emission of redispersible Er3+, Nd3+ and Ho3+ doped LaF3 nanoparticles”; and    G. A. Hebbink et al., Advanced Materials 2002, 14, No. 16, pages 1147-1150, “Lanthanide(III)-doped nanoparticles that emit in the near-infrared”
Semiconductor-based nanoparticles (“quantum dots”) have already been considered for use in bioassays. Bawendi et al., Physical Review Letters, 76, 1996, pages 1517-1520, report, for instance, FRET-effects in specifically labeled biological systems. Further, WO 00/29617 discloses that proteins or nucleic acids can be detected by means of “quantum dots” as label in (F)RET assays. U.S. Pat. No. 6,468,808 B1 and U.S. Pat. No. 6,326,144 B1 also describe biomolecular conjugates of quantum dots and their use in fluorescence spectroscopy.
(F)RET (fluorescence resonance energy transfer) and the related resonance energy transfer (RET) are based on the transfer of excitation energy from a donor capable of emitting fluorescence to an acceptor in close vicinity. With this technique it is possible, for instance with suitable fluorescent labels in biological systems, to determine distances on a molecular level in the range of from about 1 to 8 nm. The energy transferred to the acceptor can relax without emission by internal conversion (RET) and then leads only to the cancellation (quenching) of the donor fluorescence. Alternatively, the acceptor emits the accepted energy also in the form of fluorescence (FRET). These phenomena are well understood and, in the case of dipole-dipole interaction between donor and acceptor, can be explained by the theory of Förster (for instance, J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic Press, New York, 1990, pages 368-445). The energy transfer reduces the intensity of the donor fluorescence as well as its lifetime and simultaneously initiates, sensitizes, or increases the acceptor fluorescence. The efficiency of the energy transfer is dependent on the inverse 6th power of the intermolecular separation and decreases proportionally to R06/(R06+R6). R0, the so-called Förster radius characterizes that distance between donor and acceptor for which the efficiency of the energy transfer is 50%.
The F(RET) efficiency can be either determined via the fluorescence intensity of the donor with acceptor (QDA) and without acceptor (QD), respectively, by means of the equation 1−(QDA/QD) or by comparing the lifetimes of the donor in the presence (TDA) of and absence (TD) of the acceptor probe on the basis of the equation 1−(TDA/TD).
The use of “quantum dots” in bioassays suffers, however, from various disadvantages. Since the emission wavelengths of fluorescent “quantum dots” depends on the size of the particles, only a very narrow size distribution can be used. This represents a challenge for synthesis and/or size selection techniques. Moreover, “quantum dots” normally show relatively low quantum efficiencies which is caused by emission-free electron-hole pair recombinations. To overcome this deficiency, CdSe/CdS core/shell structures have been proposed wherein the CdS coating protects and enhances the photostability of the luminescent CdSe core (X. Peng et al., J. Am. Chem. Soc. 119, 1997, pages 7019-7029).
Typically, (F)RET-based assays are conducted with organic dye molecules, such as fluoresceine or rhodamine. For many applications a general drawback associated with these organic fluorescent dyes is their insufficient stability towards incident light. Their photo-toxicity can further damage biological material in the close environment. Other undesirable properties are their broad emission bands and the small stoke shifts, i.e. the difference between excitation and emission maximum, as well as the relatively narrow spectral excitation bands which require often the use of several light sources and/or complicated photo systems.
Accordingly, it is one object of the present invention to provide fluorescent inorganic materials which are particularly suitable for (F)RET-assays, in particular bioassays, and overcome the above-mentioned disadvantages.
It is a further object of the present invention to increase the (F)RET efficiency. A higher (F)RET efficiency increases the sensitivity of the method and improves for instance the signal/noise ratio.
In addition, (F)RET-based assays require donor molecules having high quantum yields (the ratio of emitted to absorbed protons) in order to increase the overall sensitivity of the assay. Therefore, it is a further object of the present invention to provide inorganic fluorescent particles having high quantum yields, which make them also particularly attractive for other applications than in bioassays.
According to a further object of the present invention, a specific process for the manufacture of these fluorescent materials is to be provided.
Finally, it is an object to provide a bioassay based on inorganic nanoparticulate materials.